BAGHDAD, Iraq – A roadside bomb destroyed the second heavily armored Bradley Fighting Vehicle in less than a week yesterday, killing two American soldiers, wounding four others, and indicating that insurgents have increased the power of the explosives they are using against American troops.

The blast came hours after gunmen in a passing car assassinated Baghdad’s deputy police chief and his son while they drove to work, part of a campaign to target Iraq’s security forces. Al Qaeda in Iraq, the group led by Jordanian terrorist Abu Musab al-Zarqawi, claimed responsibility.

American officials have cautioned that insurgents will escalate attacks in a bid to scuttle January 30 elections. After a roadside bomb struck a Bradley on Thursday and killed seven soldiers, the Defense Department warned that insurgents were increasing the size and power of their bombs.

The attack yesterday on a Bradley in southwest Baghdad followed the same pattern.

“It’s fair to say that they are afraid of the elections, they are afraid of what the outcome will be, and they want to do everything they can to derail that process because that’s just one more step toward their demise,” an American military spokesman, Lieutenant Colonel James Hutton, said. “This is probably an indication of their increase in effort and investment to derail the vote.”

The attack was one of several acts of major violence yesterday.

A suicide attacker detonated a bomb in a fake police car at a police station courtyard in Baghdad, killing at least four officers and wounding 10 during a shift change, police and witnesses said.

A roadside bombing killed three Iraqi National Guard soldiers and wounded six during a joint patrol with American troops in the restive northern city of Mosul, said an American military spokesman, Major Andre Hance. He said there were no American casualties.

In a suggestion that the insurgents were looking for new ways to intimidate voters, a rebel group posted threats in at least two towns warning it would deploy “highly trained” snipers to target voters around Iraq during the elections.

The statement, signed by the previously unknown Secret Republican Army, said 32 snipers will stalk voters outside polling in Wasit, a largely Shiite province south of Baghdad that includes Kut, Numaniyah, and Suwaiyra. It did not say how many would be sent elsewhere.

Sheik Fassal Raikan al-Gout, the governor of Anbar province, said he was aware of the circulated posters but dismissed their importance.

“We do not care about such statements,” he said. “We will continue to do our best and what we see fit to maintain security.”

A number of election officials and government leaders have already fallen victim to brutal terror attacks, and many have received death threats. The most prominent victim in recent weeks was the governor of Baghdad, Ali al-Haidari, who was slain with six bodyguards on January 4.

Yesterday, attackers shot and killed Baghdad’s deputy police chief, Brigadier Amer Ali Nayef, and his son, Lieutenant Khalid Amer, also a police officer. They were slain in Baghdad’s Dora district while traveling in a car on their way to work, Interior Ministry spokesman Captain Ahmed Ismail said.

Gunmen sprayed machine-gun fire from two cars driving parallel with the police chief’s vehicle close to his home before fleeing, police said. The two were alone in the car.

Al Qaeda in Iraq claimed responsibility for the Baghdad assassinations in a statement posted on the Internet, describing Nayef and Amer as foreign “agents.” The statement warned that other Iraqis cooperating with the American-led military would meet the same fate.

The authenticity of the statement could not be verified, but the Web site has been used by insurgent groups in the past to claim responsibility for and show videos of attacks in Iraq.

The government sought to strike back against the insurgents with its own press campaign yesterday. Prime Minister Allawi said authorities have captured 147 suspected insurgents in Iraq, including the new leader of an insurgent group.